Unfortunately I don't know how to reproduce this problem. It seems to happen with some specific customer accounts, but I'm unable to reproduce the error by configuring them in Thunderbird. It's possible that it only happens for some specific IMAP client.
What I can say is that I've backported the package from Kinetic, which has Dovecot 2.3.19, and the error is fixed for me.
Unfortunately I don't know how to reproduce this problem. It seems to happen with some specific customer accounts, but I'm unable to reproduce the error by configuring them in Thunderbird. It's possible that it only happens for some specific IMAP client.
What I can say is that I've backported the package from Kinetic, which has Dovecot 2.3.19, and the error is fixed for me.